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Capacitor after bridge rectifier in power supply Q

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bryan

Member
Hello all:

Is there any rule of thumb for the proper sized capacitor after the bridge rectifier in a regulated power supply. I will be using a LM350 as a regulator.

The required voltage will be set for approx 12v and I may push up to 3 amps. I was thinking of using a 4700uf cap rated for 25 volts. The power supply will be used to run some 12 volt muffin fans, so i don't think noise is that great of a concern.

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-=Bryan=-
 

Sherif Welsen

New Member
bryan said:
Hello all:

Is there any rule of thumb for the proper sized capacitor after the bridge rectifier in a regulated power supply...

-=Bryan=-
For every 100mA choose capacitor of 1000uf, "or 740uf as the american system"
 

Styx

Active Member
Orcad studant edition is your friend in this case.
Nice quick and easy
 

Roff

Well-Known Member
You need to maintain about 3v minimum across your regulator. This means the unregulated voltage should never dip below 15v. With 4700uF, you will get about 4v of ripple, which means the peaks of your unregulated voltage will have to be 19v. You will lose about 1.5v in your bridge rectifier, which means the peak voltage from your transformer needs to be at least 20.5v. This is 15v RMS. If your transformer secondary is rated at less than 15v, you need a bigger capacitor.
Sherif's equation will give you about a volt of ripple. If you can tolerate more, you can use a smaller capacitor.
 
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